This invention relates to investigating earth formations traversed by a borehole. More particularly, the present invention is directed to apparatus and methods for determining element concentration values for, and for further characterizing the attributes of, the formations surrounding a bore hole, by on-line processing of well logging data. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
In copending U.S. patent application Ser. No. 770,802, filed Aug. 29, 1985, and assigned to the assignee of the present invention, a method is disclosed for determining the values of formation attributes by transforming a column vector of formation elemental concentrations, into a column vector of mineral abundances. The basis for the method can be expressed by the equation: EQU (M)=(A).sup.-1 .multidot.(E)
which transforms the concentration of the elements (E) in the formation into the weight percentage of the dominant minerals (M) in the formation.
The method described in said copending application has been shown to agree reasonably well with the quantification and characterization of minerals in selected geological formations, when compared with the mineral analyses of actual core samples taken from these formations. To date, however, the method of said copending application has, in practice, relied upon constructing the column element matrix and/or the transformation matrix with entries taken at least in part from core samples or equivalent direct analyses of the formation. This is because no on-line logging techniques available to date are suitable for determining the concentration of all elements necessary for the column matrix with the required degree of accuracy.
The capital cost of drilling and evaluating a deep well, for example an oil or natural gas well, is extremely high, and for this reason considerable expense is incurred during those time intervals when drilling or production steps must be interrupted to evaluate the formation. With known analysis techniques, the concentration of some elements might be derivable from wire-line logging of the formation, but the concentrations of other elements would require the taking of core samples for analysis.
U.S. Pat. No. 3,665,195 entitled "Thermal Neutron Activation Radioactivity Logging Method" discloses a method of determining the existence and quantity of an element in a formation, by irradiating the formation with neutrons and detecting the induced gamma ray activity from the element of interest. After determining the thermal neutron capture cross section of the formation, the product of the gamma ray emission and the thermal neutron capture cross section is obtained as a quantitative indication of the element's abundance in the formation.
In the technical article "The Aluminum Activation Log" by H. D. Scott and M. P. Smith published in 1973, there is described a method for measuring the aluminum content of the formation, in order to estimate the formation shale fraction. A californium-252 source of neutrons is used in conjunction with the formation thermal neutron capture cross section to produce a continuous activation log of a borehole.
In U.S. Pat. No. 4,464,569 entitled "Method and Apparatus for Spectroscopic Analysis of a Geological Formation", a method is disclosed for determining basic formation component volume fractions. Relative spectral elemental yields indicative of the presence of chemical elements in the formation are obtained from a spectroscopic analysis of capture gamma ray spectra obtained from a neutron spectroscopy logging tool. The relative sensitivities of the logging tool to the specific minerals or to the chemical elements in the formation are determined either from core analysis or from tests run in known formations. The spectroscopic elemental yields and the relative sensitivities are then used together to determine the volume fractions of the basic formation components such as limestone, sandstone, porosity, salinity, dolomite, anhydrite, etc. Finally, the determined volume fractions are recorded on a storage medium.
Although the purpose of the invention described in the above-referenced patent is to determine formation basic component volume fractions, the technique does not require, and the patent does not disclose, a straightforward way for determining elemental concentrations, especially through the use of commonly available logging tools or modifications thereof. The method of the '569 patent takes appropriate combinations of measured yields, normalizes core data or laboratory measurements to obtain calibrated relative sensitivities and makes use of the constraint that the sum of all volume fractions is unity. Values of the volume fractions can then be found by solving the appropriate set of equations for the formation component volume fractions.
On-line techniques are available for determining relative concentrations of some formation elements. U.S. Pat. No. 3,521,064 entitled "Analysis of Gamma Ray Energy Spectrum For Constituent Identification", is representative of these methods. U.S. Pat. No. 3,928,763 entitled "Measurement of Subsurface Formation Lithology, Including Shaliness, Using Capture Gamma Spectroscopy," and U.S. Pat. Nos. 3,930,153 and 3,930,154 which contain substantially the same disclosure, teach utilization of a weighted least squares spectral fitting technique similar to that described in the '064 patent for obtaining relative elemental yields of elements contributing to a spectrum. Information bearing on the macroscopic neutron cross section is then used in a processing technique which derives further information relating to the volume fractions of the basic formation components such as sandstone, limestone, etc.
It should thus be appreciated that full realization of the significant advantages associated with the use of a transformation matrix to quantify the mineralogy from elemental concentrations cannot be readily achieved using element concentration techniques available in the prior art. In particular, known techniques and apparatus are not suitable for readily and accurately determining the concentrations of all significant elements of a formation, as needed for characterizing and quantifying the formation, by driving a single tool string through the borehole. The capability of measuring and recording the elemental concentrations and evaluating the mineralogy and other significant characteristics of the formation by means of a single tool pass, substantially immediately, would represent a significant advance in the art of subsurface formation logging.